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1. Field of the Invention
The present invention relates to pressure sensitive adhesives (PSAs)and the like, and in particular to plasticizers and tackifier extenders for pressure sensitive adhesives. More particularly the invention relates to improved pressure sensitive adhesives that incorporate enhanced polyisobutylene (PIB) modifiers therein. The invention also relates to methods for improving the tack strenghth and shear holding strength of pressure sensitive adhesives by incorporating therein an enhanced PIB modifier.
2. The Prior Art Background
Pressure sensitive adhesives are well known products having many uses, including, for example, and without limitation, use as an adhesive layer for labels, packing tape, structural tape, medical tape and floor tiles. The adhesive properties of pressure sensitive adhesives and the length of time that the tackiness of such adhesives is retained may be improved and enhanced by the incorporation in the adhesives of modifiers such as plasticizers and tackifier extenders. Plasticizers and tackifier extenders are often incorporated into the adhesive by adding the same to the adhesive formulation during the production of the adhesive. Generally speaking, tackifier extenders are substances, which may be used in conjunction with conventional tackifying resins to achieve the same degree of tack at a lower concentration of the tackifying resin. Plasticizers are substances that improve the physical characteristics of an adhesive by making it softer and more ductile.
Conventional pressure sensitive adhesives typically include a base polymer composition which may be a single polymer or a mixture of polymers, one or more plasticizers, one or more tackifying resins, and perhaps other additives such as fillers, antioxidants and solvents. For example, a high molecular weight (1,000,000 to 2,000,000 viscosity average molecular weight (VAMW), for example) polyisobutylene may also be used as a base polymer. Polyterpenes, which are usually solid, may be used as tackifying resins. Mineral oils may be used as plasticizers. Conventional polybutenes may be used as combination plasticizers and tackifier extenders. Additives may include fillers such as talc or calcium carbonate, antioxidants, and solvents.
As mentioned above, the performance of pressure sensitive adhesives may often be improved and enhanced by the incorporation therein of plasticizers and/or tackifier extenders. Conventional polybutene may often be used as both a plasticizer and a tackifier extender. Such polybutenes are simply added to the pressure sensitive adhesive (PSA) during formulation. The double bonds of these previously known conventional polybutene materials are mostly internal. That is to say, it is normal for at least about 90% of the double bonds to be at an internal position and for less than about 10% of the double bonds to be in a terminal (vinylidene) position. In addition, while the initial monomeric mixtures used to produce these conventional polybutenes may often be predominantly isobutylene, the same may also incorporates substantial quantities of monomers other than isobutylene. Thus, the conventional polybutenes that have been used as tackifiers in the past may generally be referred to as copolymers or terpolymers. For example, such conventional polybutenes often and commonly incorporate approximately 10% or so by weight of 1-butene and/or 2-butene, with only about 90% of the monomeric units in the polymer being isobutylene units.
The incorporation of conventional polybutenes into the pressure sensitive adhesive is accomplished simply by introducing the polybutene into a vessel along with all of the other components of the pressure sensitive adhesive formulation and heating and blending the mixture.
In accordance with one aspect of the concepts and principles of the present invention a new and highly useful pressure sensitive adhesive which comprises a base polymer and a modifier. The modifier may desirably be provided in the form of an enhanced PIB polymer product comprising a homopolymer of isobutylene wherein a predominant portion of the double bonds are in an alpha or beta position. The modifier may function as a plasticizer, a tackifier extender, or both. In further accordance with the preferred aspects of the invention, the enhanced PIB polymer product may desirably have a polydispersity no greater than about 2.2 and a number average molecular weight (MN) that is within the range of from about 900 to about 3000.
Preferably, in further accordance with the concepts and principles of the invention, at least about 95%, preferably at least about 96%, desirably at least about 97%, even more desirably at least about 98%, and ideally at least about 99% of the monomeric units incorporated in the PIB modifier molecules are isobutylene moieties. Even more desirably, no more than about 1% of the monomeric units incorporated in the PIB modifier molecules are something other than isobutylene moieties.
In still further accord with the concepts and principles of the invention, at least about 40%, preferably at least about 50%, even more preferably at least about 60%, desirably at least about 70%, more desirably at least about 80%, even more desirably at least about 90%, and ideally more than 90% of the double bonds of the enhances PIB polymeric modifier of the invention may be in the alpha position. In a most preferred form of the invention, no more than about 1% of the double bonds of the PIB polymer product are in a position other than alpha or beta.
Desirably, after removal of any solvent used in the formulation of the improved pressure sensitive adhesives of the invention, the same may include from at least about 10 to about 60% or more by weight of the enhanced PIB plasticizer and tackifier extender.
The invention also provides a method for improving characteristics such as the tack strength and shear strength properties of a pressure sensitive adhesive that comprises including an enhanced PIB plasticizer and tackifier extender in the pressure sensitive adhesive formulation.